The present invention relates to a rotating device provided with a dynamic pressure bearing in which a space is formed between a rotor and a lower thrust bearing and between the rotor and a radial bearing by the action of a dynamic pressure generating groove with rotation of the rotor. Further, the present invention relates to a light beam deflecting apparatus using the rotating device.
In comparison with conventional rotating devices using ball bearings, a rotating device using a dynamic pressure bearing is superior in that it makes extremely high speed rotation possible. As a result, recently, further development has been conducted for it. Generally, the rotating device using the dynamic pressure bearing is composed of upper and lower thrust bearings, a radial bearing, and a rotor rotatable on the radial bearing. With the rotation of the rotor, an air gap of several microns is formed between the rotor and the bearings by the action of dynamic pressure generating grooves provided on each of the bearings, thereby reducing the resistance between the rotor and the bearings. As a result, it allows the rotor to rotate at very high speeds. In such rotating device using the dynamic pressure bearing, since it is necessary to maintain the air gap of several microns between the rotor and the bearings, it is requested for the rotor to be extremely well balanced so as to have high grade balance. Taking an example of a rotor rotating in 20,000 rpm, its balance grade has to be not lower than G1 grade as defined in JIS BO905-1978. If its balance grade is G2 grade, vibration on the rotor become excessive. The vibration affects not only the accuracy of light beam deflection, but also causes galling and burning.
Further, ambient temperature surrounding the rotor is raised by heat generated from a motor coil during rotation of the rotor. Especially, in the case that a rotor is rotated in 20,000 rpm or more in a light beam deflecting device to deflect a laser beam in an image forming apparatus, the ambient temperature rise becomes approximate 40.degree. C. and a temperature of the rotor is also raised. Since the rotating device using the dynamic pressure bearing is required the high grade balance as discussed above, if the temperature is raised, the rotor becomes unbalance, resulting in galling and burning so that a required performance of the rotating device may not be achieved. For example, if the ambient temperature rise becomes 40 degrees, the balance grade may happen to lower from G1 grade to G2 grade, increasing vibration to an extent of ten times heavier. As a result, in the light beam deflecting device, the deflected light beam is so deviated that high quality image can not be formed.
As factors which cause the unbalance of the rotor due to temperature change, thermal expansion in the structural material of the rotor, thermal expansion in plural structural members when the rotor is composed of the plural structural members, influence of adhesive to joint the plural structural members may be listed, and they are considered to cause various thermal expansions differing in extent or location in the rotor.
Then, the objective of the present invention is to provide a rotating device or a light deflecting apparatus comprising a dynamic bearing with a structure which causes less balance fluctuation and is resistant to balance fluctuation even when the balance degree of the rotor becomes poor due to various causes,
The above objective can be attained by the following structures of the present invention.
In a rotating device provided with a dynamic pressure bearing in which a space is formed between a rotor and a lower thrust bearing and between the rotor and a radial bearing by an action of dynamic pressure generating grooves with rotation of the rotor, the rotating device satisfies the following formula: EQU (a radial bearing area)/(thrust bearing area).times.(rotor weight)&lt;300
wherein the radial bearing area (mm.sup.2) is an area of the rotor facing the radial bearing, the thrust bearing area (mm.sup.2) is an area of the rotor facing the lower thrust bearing, and the rotor weight (g) is the weight of the rotor.
In the above rotating device, on an upper section of the radial bearing is provided a preventing means for preventing the rotor from slipping out.
In the above rotating device, on the thrust bearing is formed a dynamic pressure generating groove.
In the above rotating device, on a lower section of the rotor is provided a magnet, and on a position facing the magnet is provided a coil so that the rotor is rotated together with the magnet by switching on an electric circuit for the coil.
In the above rotating device, on the rotor is provided a polygonal mirror.